This invention is in the field of audio amplifiers, and is more specifically directed to pulse-width modulated class D audio power amplifiers.
As is fundamental in the art, electronic amplifier circuits are often classified in various “classes”. For example, the output drive transistors of class A amplifier circuits conduct DC current even with no audio signal, and the entire output voltage swing is of a single polarity. Class B amplifiers, on the other hand, typically include complementary output drive transistors, driving an output voltage swing including both positive and negative polarity excursions. Class B amplifiers are necessarily more efficient, because both complementary output drive transistors are never on at the same time. Class AB amplifiers maintain a small bias current through complementary output drive transistors, so that the output voltage swing is centered slightly above (or below) ground voltage. While the non-zero bias current renders class AB amplifiers theoretically less efficient than class B amplifiers, class AB amplifiers present avoid the crossover distortion of class B amplifiers.
In recent years, digital signal processing techniques have become prevalent in many electronic systems. The fidelity provided by digital techniques has increased dramatically with the switching speed of digital circuits. In audio applications, the switching rates of modern digital signal processing are sufficiently fast that digital techniques have become widely accepted for audio electronic applications.
Digital techniques for audio signal processing now extend to the driving of the audio output amplifiers. A new class of amplifier circuits has now become popular in many audio applications, namely “class D” amplifiers. Class D amplifiers drive a complementary output signal that is digital in nature, with the output voltage swinging fully from “rail-to-rail” at a duty cycle that varies with the audio information. Complementary metal-oxide-semiconductor (CMOS) output drive transistors are thus suitable for class D amplifiers, as such devices are capable of high, full-rail, switching rates such as desired for digital applications. As known in the art, CMOS drivers conduct extremely low DC current, and their resulting efficiency is especially beneficial in portable and automotive audio applications, as well as in small form factor systems such as flat-panel LCD or plasma televisions. In addition, the ability to realize the audio output amplifier in CMOS enables integration of an audio output amplifier with other circuitry in the audio system, further improving efficiency and also reducing manufacturing cost of the system. This integration also provides performance benefits resulting from close device matching between the output devices and the upstream circuits, and from reduced signal attenuation.
FIG. 1 illustrates the architecture of a conventional class D audio amplifier, specifically a so-called “class AD” audio amplifier with a bridged load. In this conventional audio implementation, audio speaker SPKR is the load, and is bridged between pairs of output transistors 7A, 7C; 7B, 7D. Feedback from the output is also used to generate a negative feedback signal via feedback unit 9. In this architecture, input analog signal IN is received at one end of adder 11, along with this negative feedback signal. Adder 11 presents a difference signal between the input IN and the feedback signal to loop filter 13, which applies transfer function H(s) that establishes the stability of the system and also the extent to which error is suppressed by the feedback loop. The filtered difference signal from loop filter 13 is applied to an input of natural sampling pulse width modulator 1.
Conventional pulse width modulator 1 includes comparator 5, which compares its input signal from loop filter 13 with a triangle wave generated by signal source 3, and applied to the negative input of comparator 5. The triangle waveform is at a period T and a switching frequency Fsw, as shown. In this example, in which the load SPKR is bridged between output drive transistors, comparator 5 generates complementary, two-level, pulse-width-modulated (PWM) output signals. Referring to the positive output from comparator 5, which is applied to output transistors 7A and 7D, the output signal is at an amplitude of +1 (turning on transistors 7A, 7D) responsive to the filtered input difference signal being instantaneously higher than the current state of the triangle waveform, and at an amplitude of −1 (turning off transistors 7A, 7D) responsive to this input signal instantaneously being lower than the current state of the triangle waveform. The negative output of comparator 5 produces a complementary output signal, turning on and off transistors 7B, 7C. It is known to include some sort of gating or synchronization between the complementary output signals to ensure that both output drive stages are not on at the same time, thus ensuring that “crowbar” current is not drawn.
This conventional amplifier circuit is often referred to as a class “AD” amplifier arrangement, in that a zero input signal (i.e., zero difference signal between input signal IN and the feedback signal, as filtered by loop filter 13) will produce a 50% duty cycle output drive across load SPKR. This class AD arrangement is effected by transistors 7A, 7D being turned on while transistors 7B, 7C are off, so that current flows from left-to-right through load SPKR in one half-cycle, and so that transistors 7B, 7C are on while transistors 7A, 7D are off in the other half-cycle, during which current flows from right-to-left through load SPKR. In this arrangement, the common mode voltage across the bridged load SPKR is zero volts.
By way of further background, other class D amplifier arrangements are also known in the art. One such arrangement is referred to as the class “BD” amplifier, by way of analogy to class B analog amplifiers. In the class BD amplifier, the bridged load is driven by separate modulators. As a result, there are three possible drive states across the bridged load: full positive polarity, full negative polarity, and zero volts. As a result, for zero input signal, no output PWM signals appear at all (i.e., there is zero output, or the PWM output is at a “zero” state).
Class D amplifiers have become attractive for audio applications, especially as the desired output power levels have increased over recent years. The efficiency of class D amplifiers in driving loudspeakers can be higher than 90%, which is much higher than the efficiency provided by conventional analog audio amplifiers. Among other benefits of this improved efficiency, the heat that is dissipated in the drive circuitry is much reduced, and thus the amplifier heat sinks can be much smaller (and thereby lighter). Class D audio amplifiers have thus become quite popular for portable and automotive audio systems.
However, a particular problem in class D audio amplifiers is presented by the transient events of muting and un-muting of the audio system. As is fundamental in the art, a steady-state square wave time-domain signal (corresponding to a 50% duty cycle PWM signal) transforms into the frequency domain as discrete frequency components at the fundamental “carrier” frequency and its harmonics. It has been observed that if the PWM signal is abruptly gated on or off or otherwise abruptly changes its duty cycle, however, significant energy is present in sidebands to the carrier frequency and its harmonics. And even if the fundamental frequency is relatively high, the abrupt gating on or off of the PWM signal can result in sidebands with significant energy that extend into audible frequencies, which manifest as audible “clicks” or “pops”. In audio systems, this gating on and off of the PWM output occurs when the user mutes or unmutes the audio output, and at power-up and power-down, in which case the audible clicks and pops are very undesirable.
In conventional analog non-switching amplifiers, clicks and pops in the audio output are reduced by smoothing the change in biasing, for example at power-up. However, these smooth biasing changes cannot be directly applied in class D amplifiers, because of the switching operations of the output transistors. According to another conventional approach, clicks and pops are reduced by introducing a switch or relay that disconnects the load during mode changes, thus eliminating transients from appearing at the load; however, the insertion and control of such a switch or relay has proven to be cost-prohibitive, especially in modern systems.
Considering that class D audio amplifiers effectively operate in the digital realm, and also considering that many sources of audio input signals are also digital in nature (e.g., compact discs, MP3 and other digitally compressed music files, satellite radio), many modern audio systems are fully digital, in that they receive digital input signals and produce digital, PWM, class D amplifier output. In these fully digital systems, digital signal processing techniques for suppressing clicks and pops are known. Examples of these digital techniques are described in U.S. Pat. No. 6,720,825 and in U.S. Patent Application Publication No. US 2004/0017854, assigned to Texas Instruments Incorporated and incorporated herein by this reference.
However, analog-input audio systems with class D amplifier output remain important and numerous, such systems including conventional radios and especially car radios. Car radios are also vulnerable to other complexities, such as the need to operate reliably over a wide range of power supply voltages (e.g., battery or alternator voltages ranging from about 8 volts to about 20 volts), which effectively eliminate the ability to control transients through the use of precise and stable regulated voltages. These constraints complicate the design of high-performance automotive audio systems.
By way of further background, copending application Ser. No. 10/846,281, filed May 14, 2004, entitled “Improved Loop Filter for Class D Amplifiers”, commonly assigned with this application and incorporated herein by reference, describes a class D amplifier circuit in which the open-loop error for audio band frequencies is improved by a loop filter that has multiple feedback loop paths.